The present invention relates to a rolling bearing filled with grease compositions.
More particularly, the present invention relates to a rolling bearing having excellent durability and long lifetime, which is suitably applied to a device which is run under conditions of high temperature, high speed and high load and water is easy to enter into it, such as vehicle electrical component parts, an alternator and an intermediate pulley as an engine auxiliary equipment, and an electromagnetic clutch for car air-conditioner.
It is a common practice that rolling bearings are used for vehicle electrical component parts, an alternator and an intermediate pulley as an engine auxiliary equipment, an electromagnetic clutch for car air-conditioner, and the like. Grease is mainly used for the lubrication of the rolling bearings.
There is an increasing demand of size reduction, high performance and high power in the vehicle electrical component parts and the engine auxiliary equipment. In this circumstance, with the result of the size reduction, the output power reduction inevitably occurs. In the alternator and electromagnetic clutch for car air-conditioner, the power reduction is compensated by increasing their speed. In this case, the intermediate pulley also increases speed thereof and generates heat. The demands of improving the silence within a car cabin prompts sealing performance of the engine room. However, on account of temperature increasing in the highly sealed engine room, the related parts are required to endure high temperature.
On the other hand, the grease for the rolling bearing used in the related component parts generally contains a synthetic oil as a base oil and a urea compound as a thickening agent. A bearing lubricating lifetime of the urea/synthetic oil base grease is long in the temperature range up to 180xc2x0 C., and is sufficient in practical use. However, working temperature is increasing in the recent tendency of the rolling bearing, as described above. In such a condition that the bearing temperature is 200xc2x0 C. or higher, base oil may evaporates, the grease will be harden by evaporation of the base oil, and the grease will be soften by destruction of the thickening agent. Accordingly, in the case of the rolling bearing using the urea/synthetic oil base grease, seizure will occur in the rolling bearing in an early stage of a condition that the bearing temperature is 200xc2x0 C. or higher.
The rolling bearings incorporated into the component parts are used in high temperature, high speed, high load conditions. Accordingly, the grease is dissolved to sometimes generate hydrogen, as described above Japanese patent No. 2878749. The generated hydrogen invades into the bearing steel, so that the rolling bearing suffers from the flaking being attendant with a white structure caused by the hydrogen brittleness. The white structure is a type of structural changes. How to prevent the flaking is an additional important problem.
Those rolling bearings are used for the auxiliary equipment for driving the belt located outside the engine, and frequently receive muddy and rain water from the road surface. In those bearings, the invading of water into the bearing may be prevented partly by the contact rubber seal. However, it is impossible to perfectly prevent the invading of water at the present stage. The automobile engine is repeatedly driven and stopped. When the engine is at rest, temperature within the housing of the rolling bearing drops and reaches a dew point, and moisture in the air around the bearing condenses into water droplets. As a result, droplets sometimes attach to the bearings or invades into the lubricant. As disclosed in JP-A-11-72120, the water which entered into the bearing generates hydrogen, and as in the above case, the flaking being attendant with the white structure caused by the hydrogen brittleness occurs in the bearing.
The rolling bearings of the above component parts are rotated by a pulley drive system using the belt. Accordingly, static electricity is generated between the belt and the pulleys. Normally, the inner and outer rings are electrically insulated from each other by an oil film of the lubricant, and a large potential difference is present between them. When those are brought into metallic contact with each other by a strong vibration or the like, current flows, at a dash, between the inner and outer rings are conductive. The DC voltage produced electrolyzes the water, and promotes the generation of hydrogen ions. As a result, the flaking being attendant with the white structure caused by the hydrogen brittleness is more likely to occur in the bearing.
It is almost impossible to use the urea/synthetic oil base grease in a high temperature environment of 200xc2x0 C. or higher. For this reason, the grease cannot satisfy the requirement of further increasing the heat resistance.
As described above, due to the entering of water from outside, such problem arises that the rolling bearings incorporated into the component parts are likely to suffer from the flaking being attendant with the white structure caused by the hydrogen brittleness, and static electricity in the bearing promotes the flaking. How to prevent the flaking is an additional important problem. To cope with the problem, there is a proposal in which to prevent the invading of hydrogen into the bearing, an oxidized film is formed on the surfaces of the inner and outer rings and the rolling elements by blending a passivating oxidizing agent into the grease (Japanese patent No. 2878749). There is another proposal as disclosed in JP-A-11-72120. In the proposal, pH of the grease is adjusted to be within a range from 5 to 13, whereby the progress of the cathode reaction for causing the hydrogen brittleness is restrained.
However, the former proposal fails to prevent the invading of hydrogen till the oxidized film is formed. Further, it can insufficiently prevent the hydrogen invading unless the oxidized film is completely and closely formed. On the other hand, in the latter proposal, the adjusted pH value sometimes varies from the value as first adjusted. Accordingly, it is difficult to secure the stable effect.
Accordingly, the present invention has an object to provide a rolling bearing which is free from the flaking being attendant with the white structure caused by the hydrogen brittleness even when it is used under the conditions of high temperature, high speed and high load, and receives water from outside, and has a long lifetime.
To achieve the above object, there is provided a rolling bearing according to the first aspect of the present invention comprising: an inner ring; an outer ring; a plurality of rolling elements arranged rotatably between the inner ring and the outer ring; a retainer for retaining the plurality of rolling elements; and a grease composition filled in a bearing space defined by the inner ring, the outer ring, and the rolling elements; wherein the grease composition contains a base oil including of fluorocarbon polymer oil, a thickening agent including a polytetrafluoroethylene, and a conductive material, wherein an amount of the conductive material contained in the grease composition is in a range from 0.1 to 10 wt % with respect to the total weight of the grease composition.
In the rolling bearing of the present invention, the grease composition contains a base oil comprising of fluorocarbon polymer oil and PTFE. Therefore, the rolling bearing has a high heat resistance and it has high anti-seizure properties in a temperature condition of approximately 200xc2x0 C. The electrolyzing of water, which promotes the hydrogen ions generation, is prevented by permitting an electrical conduction between the inner ring and outer ring by using the conductive material. As a result, it is prevented that the flaking being attendant with the white structure caused by the hydrogen brittleness occurs.
To achieve the above object, there is provided a rolling bearing being lubricated by grease and used for vehicle electrical component parts and engine auxiliary equipment, the rolling bearing according to a second aspect of the present invention comprising: an inner ring; an outer ring; a plurality of rolling elements arranged rotatably between the inner ring and the outer ring; a retainer for retaining the plurality of rolling elements; and a grease filling the rolling bearing; wherein the grease contains a base oil and a thickening agent including carbon black.
As described above, static electricity generated between the belt and the pulley promotes the generation of hydrogen ions. However, by using the carbon black as a conductive material for the thickening agent, the grease is always put in a conductive state. Accordingly, little potential difference is present between the inner and outer rings, and there is less chance of electrolyzing water. Therefore, it is restrained that the flaking being attendant with the white structure occurs and progresses. Additionally, this effect appears immediately after the bearing is filled with the grease, and little varies with time.